lint: fix lints appearing in rust stable currently (#7118)

* [x] I have followed the instructions in the PR template

crates/egui_demo_lib/src/demo/screenshot.rs

@@ -58,7 +58,7 @@ impl crate::View for Screenshot {
                         None
                     }
                 })
-                .last()
+                .next_back()
         });
 
         if let Some(image) = image {

crates/egui_demo_lib/src/demo/tests/grid_test.rs

@@ -110,7 +110,7 @@ impl crate::View for GridTest {
             ui.end_row();
 
             let mut dyn_text = String::from("O");
-            dyn_text.extend(std::iter::repeat('h').take(self.text_length));
+            dyn_text.extend(std::iter::repeat_n('h', self.text_length));
             ui.label(dyn_text);
             ui.label("Fifth row, second column");
             ui.end_row();